Two-part adhesive systems typically consist of a first part which comprises a solution of at least one polymerizable monomer, e.g., an acrylic or methacrylic ester, and a free radical initiator, and a second part which comprises an accelerator for the initiator. The adhesive system is cured by polymerization of the mixture by a redox reaction or free radical mechanism. The free radicals are generated by reaction of the initiator, i.e., an oxidant, with the activator or catalyst, i.e., a reductant. Typically, the rate of radical generation, which determines the rate of polymerization or speed of cure, depends in part upon the nature of the redox reaction.
Key factors in performance of these adhesives include resistance to impact and stress, high adhesion to substrates, resistance to environmental attack or degradation, and rate of cure. It is often desirable that the joint parts, one coated with adhesive and the other coated with activator, cure as quickly as possible after being joined together such that the strength of the joint develops rapidly. The more rapid the cure, the less clamp or fixture time required, and, in general, the greater economic advantage offered, particularly in high volume manufacturing processes. Further, in the case of two-part adhesives, the useful lifetime, or "on-the-part life", of the activator is important. It would typically be desirable to prime, i.e., apply activator to, a surface in anticipation of later contact to or assembly with the opposing, adhesive-coated, surface well in advance of assembly time in order to permit greater facility in manufacturing operations.
U.S. Pat. No. 3,994,764 (Wolinski) discloses a redox reaction-curable adhesive composition employing benzoyl peroxide initiator and tertiary amine, e.g., dimethylaniline, activator. The composition optionally also includes an oxidizable heavy metal in the activator. Dimethylaniline is volatile and relatively toxic, however, leading to preference for use of a variety of other aromatic amine derivatives as activators. In some of those cases, however, the odor of aromatic amines is unacceptable to the end user.
U.S. Pat. No. 3,890,407 (Briggs, Jr. et al.) discloses the use of condensation products of amines, e.g., aniline, and aldehydes, e.g., butyraldehyde, as accelerators or activators for sulphonated adhesive compositions. U.S. Pat. No. 4,182,644 (Briggs, J. et al.) discloses the use of an accelerator which may be condensation reaction product of an aldehyde, e.g., butyraldehyde, and a primary amine in adhesive compositions comprising a specified polymer-in-monomer solution, an organic sulfonyl chloride having up to 20 carbon atoms and/or chlorosulfonated polymer, and polymerization catalyst.
U.S. Pat. No. 3,870,675 (Kusayama et al.) discloses a two-component adhesive consisting of (1) at least one defined divinyl compound, and at least one rubber material, and (2) an accelerator comprising a thiourea compound, at least one of copper salts, cobalt salts, manganese salts, chromium salts, and vanadium salts of organic or inorganic acids, and an organic volatile solvent.
U.S. Pat. No. 3,970,505 (Hauser et al.) discloses an anaerobic adhesive composition which contains acrylate ester monomers, peroxy initiators, and as an activator, a substituted thiourea and an acidic compound. U.S. Pat. No. 4,554,301 (Dohi et al.) discloses the use of thiourea compounds such as dibutyl thiourea or ethylene thiourea with metal chelate compounds such as vanadyl or cobalt acetylacetonate or metal soaps such as manganese or cobalt naphthenate as accelerators.
U.S. Pat. No. 4,569,976 (Zimmerman et al.) discloses an accelerator comprising a monosubstituted thiourea having a heteroatom in the beta or gamma position relative to the nitrogen bearing the substituent.
U.S. Pat. No. 3,616,040 (Toback) discloses the use of a bonding accelerator comprising the condensation product of an aldehyde and a primary or secondary amine.
U.S. Pat. No. 3,591,438 (Toback et al.) discloses a bonding accelerator containing the condensation product of an aldehyde and a primary or secondary amine used with a reducing activator which is either a sulfur-containing free radical accelerator or a compound containing an oxidizable transition metal.
U.S. Pat. No. 4,170,612 (Pastor et al.) discloses pressure-sensitive adhesive compositions prepared from a mixture of a specific pressure-sensitive polymer, a chlorosulphonated polyethylene, and a polymerizable vinyl compound which can be cured upon contact with a condensation reaction product of a primary or secondary amine and an aldehyde.